1. Field of the Invention
This invention relates to a method of preparing a high grade titanium dioxide (TiO2) product from titania slags by removing alkaline earth and other impurities usually found in slags. The method of the present invention generally comprises the steps consisting of sizing the slag, oxidizing it at high temperature, reducing the resulting material at high temperature, subsequently acid leaching the reduced material at elevated temperature and pressure to yield an upgraded slag product and a leachate, and finally calcining the leached product. The upgraded slag obtained from the inventive method is a suitable feedstock for the chloride process of TiO2 pigment production.
Optionally, the upgrading process may also comprise a caustic leaching step performed immediately after the acid leaching step. The caustic leaching step will be particularly useful to remove residual SiO2 in the upgraded product.
2. Description of the Prior Art
Titanium Feedstocks for TiO2 Pigment Production
The present invention is directed to a process for the upgrading of titania slags into a product having a very high TiO2 content with low levels of alkaline-earth and other impurities.
Titanium is the ninth most abundant element in the earth""s crust. Of the various titanium based products, titanium dioxide (TiO2), holds the greatest industrial and commercial significance. It is a high-volume chemical in most of the industrialized world. Titanium dioxide is used as pigment in paints, plastics, papers, inks, etc.
Titanium dioxide (TiO2) is commonly found in nature in the form of xe2x80x9cilmenitexe2x80x9d ores containing from 30 to 65% TiO2 in association with varying amounts of oxide impurities of the elements iron, manganese, chromium, vanadium, magnesium, calcium, silicon, aluminum and others. Ilmenite ores are commercially upgraded into titania xe2x80x9cslagxe2x80x9d containing typically 70-90 wt % TiO2 by electro-smelting processes conducted at very high temperatures (molten state) in electric arc furnaces. Ilmenite ores are also upgraded into xe2x80x9csynthetic rutilexe2x80x9d products containing 92-95 wt % TiO2 by processes consisting in the xe2x80x9cleachingxe2x80x9d of ilmenite ores with mineral acids or in reducing the iron oxide impurities in the presence of coal at moderately high temperatures (solid state reduction) in rotary kiln type furnaces. xe2x80x9cRutilexe2x80x9d is a still richer form of TiO2 (93-96% TiO2) which occurs naturally but is rarely found in deposits of commercial significance.
The production of TiO2 pigments is based on two processes. The traditional xe2x80x9csulfatexe2x80x9d process consists in solubilizing ilmenite or slag by dissolving it in concentrated sulphuric acid; pure TiO2 is then obtained by selective hydrolysis of the liquors containing the solubilized TiO2. In the newer xe2x80x9cchloridexe2x80x9d process, a feedstock such as ilmenite, slag, synthetic rutile or natural rutile is fluidized at high temperature (typically 950-1200xc2x0 C.) in a stream of chlorine gas to produce a vapour mix of chlorides, including TiCl4 and the chlorides of the feedstock impurities; TiCl4 is separated from the impurity chlorides by selective condensation and is subsequently converted to pure TiO2 by contacting it with oxygen at high temperatures (chlorine gas is recovered in the oxidation treatment).
The main technical requirement for sulfate process feedstocks is that these must be soluble in concentrated sulphuric acid. For the chloride process, however, the main technical requirements are: i) the feedstock must contain low concentrations of alkaline-earth oxides such as MgO and CaO, and ii) the particle size range must be compatible with the fluid bed equipment used to chlorinate the feedstock. In addition, environmental and economic considerations dictate the need for the highest possible TiO2 contents in the feedstock.
The present invention relates specifically to the preparation of a high grade TiO2 feedstock suitable for the fast growing chloride pigment process by upgrading titania slags. The initial slag can be naturally low in alkaline-earth oxide impurities, such as the slag produced from ilmenite ores mined in the East Coast of the Republic of South Africa, or could contain higher levels of these impurities, as is the case of slag produced from ilmenite ores mined in the Province of Quebec, Canada. In both cases the resulting upgraded product is of similar TiO2 contents (typically 94-96% TiO2) and exhibit contents of alkaline-earth oxides well below the maxima generally acceptable for chloride feedstocks (1.5% MgO and 0.20% CaO) This is an important aspect of the invention since the use of slags containing higher levels of alkaline-earth oxides has been up to now restricted to the sulfate pigment process.
Oxides of the alkaline earth metals such as MgO and CaO are undesirable in the chloride pigment process as they form during chlorination paste-like condensates of MgCl2 and CaCl2 which tend to foul the fluidizing reactors and other downstream equipment. However, alkaline-earth oxides are commonly found in magmatic TiO2-bearing deposits known as rock ilmenites which represent the most abundant sources of TiO2. Rock ilmenites, being relatively low in TiO2 contents (30-45% TiO2) but containing high concentration of iron oxides, can only be economically upgraded by electro-smelting processes which produce a titania slag and recover the iron values in the form of high purity iron products, the latter feature not being possible in other commercial ilmenite upgrading processes. While electro-smelting of rock ilmenites renders the resulting slag suitable as a feedstock for the sulfate process, the smelting does not remove sufficient amounts of impurities, such as alkaline-earth impurities, including magnesium and calcium, to make the slag suitable as a feedstock for the chloride process.
There is therefore a need to provide a commercially attractive method for further upgrading slags obtained from ilmenites, including those ilmenites naturally high in alkaline-earth impurities, to yield a suitable high grade feedstock for the chloride process of TiO2 production.
Unexpectedly, it has been discovered that titania slags can be treated in a novel and commercially efficient process to produce an upgraded slag product which is an excellent feedstock for the chloride process.
Differences Between Slags and Ilmenites
The literature contains a number of prior art processes aimed at the upgrading of ilmenite ores into synthetic rutile type products by applying mineral acid leaching techniques.
These processes are not applicable to the upgrading of titania slag because of the vastly different chemical and physical nature of ilmenite ores and titania slags. As will be shown in the figures which form part of this application, it is manifest that the X-ray diffraction patterns of ilmenite ores and slags are quite different indicating that their chemical and physical properties are also quite different. What follows is a description of the chemical and physical differences separating ilmenite ores from titania slags.
Ilmenite ores are found in nature as primary ilmenites (FeTiO3) or weathered ilmenites and mixtures thereof. Weathered ilmenites result from oxidation by ground water which gradually transforms primary ilmenites through the following major phases: pseudorutile (Fe2.3Ti3O9), altered pseudorutile (Fe1.2Ti3O6.6(OH)2.4), leucoxene (Fe0.6Ti3O4.8 (OH)4.2) and finally natural rutile (TiO2). The prior art has evolved various processes for upgrading ilmenites (primary, secondary and mixtures thereof) to synthetic rutile by concentrating the TiO2 content and removing iron as well as various gangue minerals and other impurities by mineral acid leaching processes. These prior art processes, which will be discussed in greater detail below, are usually adapted for use with ilmenites and do not yield satisfactory results with titania slags mainly because slags are physically and chemically different from ilmenites.
Titania slags are generally produced by reduction smelting of ilmenite ores in an electric arc furnace. The resulting slags consist of two main phases:
(i) an abundant pseudobrookite phase which can be described as a solid solution of different titanates and whose general formula is as follows:
(FeTi2O5)a (MgTi2O5)b (Al2TiO5)c (MnTi2O5)d (V2TiO5)e (Ti3O5)f
wherein a+b+c+d+e+f=1.
Such crystallographic phase is not known to occur naturally in the earth""s crust, although a similar crystalline association known as armalcolite has been found in lunar rocks brought back by the Apollo missions.
As an example, the pseudo-brookite phase constituting the bulk of the commercially available SORELSLAG(trademark) can be described by the following formula:
(FeTi2O5)0.31 (MgTi2O5)0.30 (Al2TiO5)0.06 (MnTi2O5)0.08 (V2TiO5)0.012 (Ti3O5)0.31
Such phase contains practically all of the titanium found in the slag and most of the iron, magnesium, manganese, vanadium and certain other impurities found in the slag.
A notable feature of this phase is its inherent inertness toward the action of mineral acids relative to titanium-bearing phases present in ilmenite ores. Such inertness renders the slag very difficult to upgrade by acid leaching processes, unless its structure is substantially converted into formations more amenable to the leaching action of such acids.
(ii) a minor glassy silicate phase is present in the form of inclusions, attachments and veins inside the pseudobrookite phase. The general formula is as follows:
(Ca, Al, Mg, Fe, Ti)SiO3.
A typical chemical composition of this glassy silicate phase is as follows when expressed in % wt:
It is observed that most of the CaO impurity is concentrated in this glassy silicate phase which is rather impervious to leaching. The CaO content is a tenacious alkaline-earth impurity which must be removed or at least significantly reduced if it is hoped to produce an upgraded slag product suitable for the chloride pigment process. Thus, it is important to find a way to decompose this glassy silicate phase to free the CaO for subsequent leaching.
It is noted that such glassy silicate phases are characteristic of titania slag and are generally absent in ilmenite ores. Furthermore, the prior art does not teach any efficient means for the physical separation of the glassy silicate from slags.
From a physical point of view, titania slags are produced in the molten state and are usually cast in ladles or similar equipment to produce solid blocks ranging in weight from a few tons to 30-40 tons. This contrasts with ilmenite ores, used for the production of synthetic rutile by acid leaching processes, whose natural grain size is typically in the 75-250 micron range. It follows that titania slag must be initially sized by means of crushing, screening and classification technologies prior to subjecting it to an upgrading process.
It should be noted that the slag sizing process offers an opportunity to tailor the size distribution of the feedstock to the optimum requirements of the chloride pigment process. In the present invention, the initial titania slag is preferably sized between 75 and 850 microns with a mean particle diameter (d50) in the range of 250-350 microns. It has been found that such size distribution enhances the productivity of the fluid bed chlorination reactors while reducing the process losses due to entrainment of very fine particles in the stream of gaseous chlorides produced in the reactors.
In summary, a process for the upgrading of titania slag will differ from prior art processes for the upgrading of ilmenite ores, inter alia, in the following regards:
i) sizing of the slag is required;
ii) extensive modification of the titanium-bearing pseudo-brookite phase of the slag is required to facilitate the action of mineral acids for the removal of impurities such as iron, magnesium, manganese, vanadium, aluminum and others;
iii) extensive modification of the calcium-bearing glassy silicate phase of the slag is required to facilitate the removal of calcium if such element is present in excess of the levels that are tolerable in the chloride pigment process.
iv) acid leaching of the slag is conducted under specified conditions of temperature, pressure, acid concentration, time and other process variables.
Prior Art Processes
The literature contains a number of processes to upgrade titania slags into high TiO2 products suitable as feedstocks for the chloride process of pigment production. Thus, Guxc3xa9guin in U.S. Pat. Nos. 4,933,153, 5,389,355 and 5,063,032 proposes to:
i) partly upgrade the slag by contacting it with chlorine gas at moderate to high temperatures, and
ii) subsequently leach the partly upgraded product with hydrochloric acid in pressure vessels.
In U.S. Pat. No. 4,629,607, Guxc3xa9guin also discloses a method consisting in the partial chlorination of pre-heated slag which does not include a subsequent acid leaching step. Such method is not effective in removing alkaline-earths impurities and its application is therefore more useful for the upgrading of slags naturally low in these types of impurities.
U.S. Pat. Nos. 4,120,694 and 4,362,557 (Elger et al.) disclose processes for the removal of MgO and CaO impurities from finely ground and pelletized titania slag by sulfonation roasting using SO3 at a temperature range of 600-1000xc2x0 C. in order to form a more easily removable double sulfate, i.e. CaSO4*3MgSO4. Sulfonation promoters such as sodium salts are also proposed. However, the processes require much time (upwards of 20 hours) to sufficiently reduce the MgO and CaO content for its intended use and do not efficiently remove other impurities, generally yielding a product which must undergo further treatment prior to use as a feedstock in the chloride process of TiO2 production.
In contrast to the above disclosures, the process disclosed herein achieves the necessary modification of the slag structure by means of simpler treatments consisting in the sequential oxidation and reduction of the slag conducted under specified thermodynamic and retention time conditions. The treated slag is then subjected to an acid leaching step conducted under practical conditions of temperature, pressure and contact time.
The prior art also proposes various other processes which may include acid leaching steps but which are specific to the upgrading of ilmenite ores. Indeed, these processes are mostly directed to the removal of the iron oxide impurities, since other impurities, notably MgO and CaO, but also others such as Al2O3, V2O5, etc. are generally absent or present in small concentrations in the ilmenite ores which are the object of the prior art disclosures. In addition, the prior art processes are designed to deal with mineralogical structures which are substantially more amenable to the leaching action of mineral acids than those found in titania slags. It is noteworthy that some of these prior art processes include certain unit operations which resemble certain portions of the present disclosure. However, as will be illustrated later by the way of examples, when these prior art processes are applied to titania slags, they fail to produce the results obtained by applying the process of the present invention.
For example, Sinha et al. describe in G.B. patent No. 1,225,826 a process for the upgrading of ilmenite ores which includes thermal treatments of oxidation and reduction generically similar to those described in the present disclosure but which are conducted under conditions of temperature and retention time that are inadequate for the successful modification of the mineralogical structure of slags. Similarly, the leaching step included in the G.B. patent No. 1,225,826 is conducted at or nearly atmospheric pressure, a condition that has been shown to be insufficient when applied to slags.
U.S. Pat. No. 3,825,419, Chen, assigned to the Benilite Corporation of America, describes yet another process for the upgrading of ilmenite which includes relatively mild oxidation and reduction treatments conducted in kiln-type furnaces and mostly aimed at reducing the trivalent iron ions to divalent ones as the trivalent iron is undesirable for the subsequent leaching of the ilmenite ore. Again, the process conditions described in this patent are inadequate for the object of modifying the structure of slags.
U.S. Pat. No. 4,199,552, Rado, describes another process for the upgrading of ilmenite ore which includes, sequentially, reduction of the ore to convert trivalent iron to bivalent iron and some metallic iron, and oxidation of the reduced ore to convert the metallic iron to bivalent iron without excessive production of trivalent iron, followed by acid leaching. Again, the process conditions described in this patent are inadequate for the object of modifying the structure of slags.
What can be learned from the prior art discussed above is that there are numerous known approaches for beneficiating ilmenite ores which may comprise oxidation, reduction or leaching steps to leach out impurities and concentrate the TiO2 content of the ore. In such processes, the iron content of the ilmenite is generally separated from the titanium by dissolving the iron as a soluble salt of the acid. However, such processes do not work with titania slag which is substantially more inert to the leaching action of mineral acids because of its high pseudobrookite content and because of its glassy silicate content. In particular, it has been observed that most of the MgO is contained in the pseudobrookite phase and that most of the CaO is found in a glassy silicate from both of which these alkaline-earth metal oxide impurities are very difficult to leach under practical conditions of pressure and temperature. Consequently, the prior art processes for upgrading ilmenites to synthetic rutile fail to address the difficulties surrounding the removal of impurities from slag.
Indeed, it has been discovered that titania slag requires a pretreatment within an unexpected window of process conditions to render it suitable for acid leaching. The pretreatment of the present invention achieves a surprising phase change in the particle structure of the slag which greatly facilitates the subsequent leaching step. Indeed, in accordance with the present invention, the very difficult to leach pseudobrookite phase of the slag is in major part shifted to a more easily leachable ilmenite-geikielite solid solution created during the process which exhibits a marked tendency to concentrate the MgO impurity. Meanwhile, the CaO impurity concentrated in the glassy silicate phase is also freed for ease of leaching by a decomposition of the glassy silicate phase.
It is therefore the primary object of the present invention to provide an efficient and economically feasible process to upgrade titania slag into a high grade product suitable for the chloride process of pigment production.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.
The process of the present invention is therefore aimed at concentrating the TiO2 content and removing impurities from a titania slag. Another way to generally describe the inventive process is a method to upgrade titania slag by effecting a pretreatment on the slag to provide an intermediate product which is more easily leached of its impurities.
In general terms, the present invention provides a method to upgrade titania slags to obtain a high TiO2-containing product having residual impurity content and grain size distribution suitable for use as a feedstock in the chloride process of titanium dioxide pigment production, said titania slag containing impurities in the form of oxides of the elements iron, manganese, chromium, vanadium, aluminum, silicon, alkaline-earths and others distributed in a pseudobrookite phase and a glassy silicate phase, the method comprising:
(a) sizing the titania slag such that the size of individual slag particles are in the 75 to 850 micron range, preferably having a mean particle diameter of about 250-350 microns;
(b) oxidizing the sized slag by contacting the slag with an oxygen containing gas at a temperature of at least about 950xc2x0 C. for a period of at least about 20 minutes such that a substantial portion of the iron oxides are converted to the ferric state, such that the reduced titanium oxides are converted to the tetravalent state, and such that at least a major portion of the glassy silicate phase is decomposed;
c) reducing the oxidized slag in a reducing atmosphere at a temperature of at least about 700xc2x0 C. for a period of at least about 30 minutes such that the ferric state iron oxides are converted to the ferrous state;
(d) mineral acid leaching of resulting treated slag at a temperature of at least 125xc2x0 C. and under a pressure in excess of atmospheric pressure to yield an upgraded leached slag product and a leachate;
(e) washing and calcining the upgraded leached product by heating such product at 600xc2x0 C. to 800xc2x0 C.
The method of the present invention thus eliminates most of the impurities contained in the original slag, including the alkaline-earth metal oxides, with minimal loss of titanium values and degradation of the size of the grains. Preferably, the upgraded slag product will contain at least 90%wt of titanium dioxide and less than 1%wt of magnesium oxide and less than 0.2%wt of calcium oxide.
It is also important to note that during the treatment steps (b) and (c), the MgO content of the slag tends to migrate to an ilmenite-geikielite phase from which it is clearly easier to leach-out the MgO. Furthermore, during oxidation step (b), the CaO, which was initially trapped in the glassy silicate phase is liberated by the decomposition of the glassy silicate.
In an optional embodiment, the method of the present invention also comprises a caustic leaching step performed after acid leaching step d) and prior to calcination step e).
The present invention provides a novel product particularly suitable for use as a feed material for the chloride process of pigment production.
Also in an optional embodiment, the method of the present invention may be abbreviated to steps a) to c), inclusively. The resulting intermediate product may be sold and used for further processing by eventual purchasers.